A passive optical network (PON) is a point-to-multipoint optical fiber network having no active (i.e., powered) components in the inter-node portion of the network. Fiber optic networks are becoming increasingly common because of the many advantages of optical fiber over standard electrical cables, such as increased bandwidth and low signal degradation.
Many users of such networks require extremely high reliability of the network. That is, the network must be operational an extremely high percentage of the time. Such users requiring very high network reliability might include the military, banks and other financial institutions, and civilian air traffic control systems. In general, communications over a network can be interrupted by two general types of failures, namely, a fiber break failure and an interface failure. As used herein, the term interface failure refers to a failure at the interface equipment of a network terminal of the network.
Of particular interest in the present specification are point-to-multipoint networks. The term "point-to-multipoint" refers to a network architecture in which all communications between nodes are routed through a control node, typically termed the head end. In this specification, the control node is termed the head end and all other network nodes are termed network terminals.
Point-to-multipoint networks may take on various configurations including a tree configuration, a bus configuration, a star configuration, and a ring configuration. They also may use any type of communication protocol, including time division multiple access (TDMA) protocols, code division multiple access (CDMA) protocols, contention protocols (e.g., CSMA-CD used for Ethernet), etc. An example of an ATM passive optical network (APON) using TDMA is described in ITU-T G983.1.
In order to provide extremely high reliability, such networks typically employ redundant architectures. For instance, in order to assure high reliability against fiber breaks, a network would be designed to provide two separate and independent fiber routes between each network terminal and the head end.
To provide protection against interface failures, each node of the network, including the head end, would be provided with redundant interfaces. Thus, if one interface failed, the node could switch to use the other interface.
Generally, it has been believed that, for networks requiring fiber and/or interface protection (i.e., redundancy), a ring architecture is most efficient. However, protected star and other networks are known.
FIGS. 1A, 1B and 1C illustrate unprotected star, tree and bus optical network architectures, respectively.
Commonly, the multiple fibers connecting a network terminal to the head end in a protected network are routed over geographically different routes. This is because the cause of a fiber break frequently is a localized event, such as severe weather, insurrection, accidental human breakage (for instance, due to construction), etc.
Examples of redundancy/protection schemes can be found, for example, in appendix D of ITU-T G983.1. J. L. De Groote, D. A. Buise, H. K. Dedecker, F. M. Louagie and H. F. Slabbinck, Redundancy and Protection--Switching in APON Systems, Broadband Access and Technology, W. Faulkner and J. L. Hammer (IDS.), 1999 also discloses several architectures for protected APONs.
FIGS. 2, 3 and 4 illustrate some of the protected APON architectures disclosed in the aforementioned article. For instance, FIG. 2 illustrates a partial protection scheme in which the head end and the fibers 28 and 30 between the head end and the splitter 26 is protected. Particularly, the head end 20 includes two interfaces 22 and 24 to the optical network. Each of those interfaces is coupled to a 2:N splitter 26 via a fiber 28 and 30, respectively, where N is the number of network terminals. The splitter 26 couples to each of the terminals, e.g., terminals 32-1 and 32-N through a fiber, e.g., 34-1 and 34-N and an interface, e.g., 36-1 and 36-N, respectively. A failure of one of the interfaces 22 or 24 at head end 20 or in fibers 28, 30 is non-fatal since the other interface can take over. However, this scheme provides no protection for failure of an interface of one of the network terminals 32-1 through 32-N. Also, it does not provide protection for any fiber breaks other than in fiber portions 28 and 30.
FIG. 3 illustrates a fully redundant, i.e., fully protected, APON network architecture. In this architecture, the head end 40 includes two optical interfaces 42 and 44. Each optical interface 42 and 44 is coupled via a fiber 46 and respectively, to a 1:N optical splitter, 50 and 52 respectively. Each optical splitter 50 and 52 is coupled to each network terminal 54-1 through 54-N via a separate fiber 56-1 through 56-N and 57-1 through 57-N and optical interface 58-1 through 58-N and 60-1 through 60-N at the terminal. For instance, splitter 50 is coupled to network terminal 54-1 via fiber 56-1 and interface 58-1. Splitter 50 is coupled to network terminal 54-N via fiber 56-N and interface 58-N. This configuration provides full redundancy for interface failure at any of the network terminals and the head end as well as for a fiber break anywhere in the network.
FIG. 4 illustrates a third protected network topology. Whereas FIGS. 2 and 3 illustrate star network topologies, FIG. 4 discloses a ring network topology. FIG. 4 shows a route redundant architecture having optical interface protection at the head end and full fiber break protection. It does not have optical interface protection at the terminals. Particularly, FIG. 4 illustrates a route redundant architecture with different drop sections along a ring network. The head end 80 has redundant optical interfaces 82 and 84 with each of the optical interfaces 92 and 84 coupled to a 1:K splitter 83, 85 with each output fiber 90, 92 and 94 forming a ring between the two splitters. K is the number of drop sections and therefore also the number of fibers. Each fiber 90, 92 and 94 couples to one or more network terminals 96, 98 or 100 through one of the drop sections. Each drop section includes a 2:M splitter 102, 104 and 106, where M is the number of network terminals coupled to the ring via that splitter. Accordingly, each group of network terminals coupled to a 2:M splitter can communicate on the ring in either the clockwise or counterclockwise direction.
In this ring architecture, the network is fully protected against optical interface failure at the head end or fiber failure anywhere between the drop sections (i.e., the 2:M splitters) and the head end. There is no protection, however, for optical interface failure at the network terminals or in the fiber sections between the network terminals and the 2:M splitters, e.g., fiber sections 112, 114.
Providing multiple fiber routes between network terminals and the head end is expensive, particularly when the fibers are laid along different routes. Further, providing redundant optical interfaces is expensive since interface equipment costs are doubled. Further, there is additional design and equipment costs associated with the circuitry and software that must be provided for switching between the redundant interfaces.
Accordingly, it is an object of the present invention to provide a low cost protection scheme for a point-to-multipoint optical network.
It is a further object of the present invention to provide a network that is fully protected against fiber breaks without the need for redundant fibers.